Method for solving unqualified head and tail performance of hot-rolled steel coil

Abstract

The present application relates to a kind of methods for solving the head and tail performance of hot-rolled steel coil unqualified, the method includes the following steps: step one, molten iron desulfurization, step two, desulfurization qualified molten iron is poured into converter along with scrap steel, step three, molten steel of converter tapping is sent to LF furnace and is refined, step four, the molten steel of step three is sent to RH furnace and is treated, step five, the molten steel of step four is sent to continuous casting process, and slab is formed, step six, the slab of step five is sent to hot rolling mill heating furnace, step seven, the slab of step six is carried out roughing, step eight, the billet of step seven is carried out finish rolling, step nine, the finish rolling material of step eight is coiled, step ten, the product coil of step nine is sampled, and the performance and metallographic structure of the head and tail of steel coil are detected, to ensure that the head and tail performance of steel coil is qualified.The scheme solves the problem of the head and tail area performance of hot coil unqualified, the method is easy to implement, and the control relationship formed is accurate and easy to use.

Description

Technical Field

[0001] This invention relates to a method, specifically a method for solving the problem of substandard performance at the beginning and end of hot-rolled steel coils, belonging to the field of hot-rolling production process technology. Background Technology

[0002] When hot-rolled steel coils are coiled, the rapid cooling rate at the beginning and end (approximately 20 meters in length) leads to substandard performance in these areas (primarily low tensile strength and yield strength). This is especially true for high-titanium steel, where the rapid cooling at the beginning and end results in a large amount of titanium dioxide (TiN) precipitation with coarse grains, weakening precipitation strengthening and causing frequent instances of substandard performance at the beginning and end of the coil. This significantly impacts the overall performance stability of the steel coil.

[0003] Currently, the existing technology for addressing the performance defects at the beginning and end of hot-rolled steel coils involves cutting off these areas, rather than taking technical measures to improve their performance. There are two reasons for this: First, some companies have considered optimizing the cooling water to solve the problem, but this has resulted in substandard impact resistance. Second, some companies haven't considered optimizing the coiling cooling water or other improvement methods. Therefore, there are no reports in the current technology regarding improvement methods for addressing performance defects in the beginning and end areas of hot-rolled steel coils.

[0004] Relying on cutting off the beginning and end areas of hot-rolled steel coils would waste a significant amount of material, especially for high-titanium steel coils. Wasting the beginning and end areas would require cutting off approximately 40 meters of material, reducing the yield of finished products. Therefore, technical measures must be implemented to address the issue of substandard performance at the beginning and end of hot-rolled steel coils.

[0005] This technology addresses the issue of substandard performance in the head and tail areas of hot-rolled steel coils, particularly high-titanium steel coils. It involves optimizing the hot-rolling coiling temperature (achieved by optimizing the cooling water volume) and the thickness of the intermediate billet. This combination of techniques effectively solves the problem of substandard performance in the head and tail areas of hot-rolled steel coils, especially high-titanium steel coils. Summary of the Invention

[0006] This invention addresses the technical problems existing in the prior art by providing a method for solving the problem of unqualified performance at the beginning and end of hot-rolled steel coils. This method is easy to implement, and the resulting control relationship is accurate and easy to use.

[0007] To achieve the above objectives, in addition to improving the relevant hot-rolling production process, research has also found that strict control of inclusions in molten steel during each step of steelmaking is crucial for ensuring stable performance at the beginning and end of hot-rolled coils. Therefore, relevant improvements have been made to the control of inclusions in molten steel during the steelmaking process. Thus, the technical solution of this invention is as follows: a method for solving the problem of unqualified performance at the beginning and end of hot-rolled steel coils, the method comprising the following steps:

[0008] Step 1: Desulfurization of molten iron to meet desulfurization requirements and slag removal requirements. To improve the performance of hot-rolled coil heads and tails, sulfide inclusions are controlled, with sulfur content controlled at ≤0.005%. The slag removal area of ​​the molten iron should be no less than one-third of the liquid surface.

[0009] Step two involves pouring the desulfurized molten iron, along with scrap steel, into the converter and adding appropriate slag-forming materials for converter steelmaking, refining the steel to the required composition and tapping it at the desired temperature. The purpose of this step, besides ensuring the required chemical composition and temperature of the molten steel, is to control inclusions to improve the performance of the hot-rolled coil ends. Therefore, the converter low-blowing process uses a nitrogen-first, then argon gas stirring method. This ensures a reduction in nitrogen atoms in the molten steel, maintaining its toughness, while also reducing the cost of argon. Aluminum deoxidation is used at tapping, and to control the oxygen content in the molten steel, the oxygen level is required to be below 500 ppm (previously required to be below 600 ppm).

[0010] Step three: The molten steel from the converter enters the LF furnace for refining. The purpose is to desulfurize and raise the temperature to meet the requirements for the next process. To ensure the performance of the hot-rolled coils at both ends, this step further controls inclusions such as sulfides and oxides (because steelmaking involves many processes and takes a long time, further control of inclusions in the molten steel is necessary, so the previous control methods alone are insufficient). The desulfurization dosage needs to be appropriately increased from the original 2200 kg / heat to 2250 kg / heat, thereby controlling the sulfur content to ≤0.003% (originally ≤0.005%).

[0011] Step four involves sending the molten steel from step three to an RH furnace for further processing. This process involves incorporating titanium and manganese alloys, as well as degassing and refining the steel. Besides alloying and vacuum degassing as required, this step also requires careful control of the oxide content in the molten steel. Production monitoring has shown that meeting the original vacuum treatment requirements for the molten steel is beneficial for improving the head and tail properties of the hot-rolled coils. Therefore, the pure degassing time for the molten steel must be greater than 8 minutes, and the vacuum level must be maintained below 26 kPa.

[0012] Step five involves sending the molten steel from step four to the continuous casting process to form slabs. Besides solidifying the steel into slabs for rolling into hot-rolled coils, the most crucial step in this process is controlling inclusions in the molten steel. Measures include proper protective casting to prevent the molten steel from being exposed to air during casting. Argon gas is used throughout the casting process, along with argon blowing through the submerged entry nozzle and stopper rod. The flow rate is controlled to be less than 7.5 L / min (excessive flow will cause large fluctuations in the liquid level in the crystallizer, which can actually introduce inclusions).

[0013] Step six: Send the slab from step five to the hot rolling mill heating furnace to heat the slab to the required rolling temperature. The slab exiting the furnace should be above 1250°C.

[0014] Step 7: Perform initial rolling on the slab from Step 6 to form an intermediate slab of the required thickness, which is 40mm.

[0015] Step 8: The intermediate billet from Step 7 is finished rolled to produce qualified steel coils that meet the user's requirements. The finishing rolling temperature is controlled at 890±20℃.

[0016] Step nine involves coiling the finished rolled material from step eight to form the product steel coil and to optimize the coiling temperature at the beginning and end of the coil, thereby improving the performance of the beginning and end of the coil. The coiling temperature at the beginning and end of the hot coil is controlled at 920±20℃, and the coiling temperature at the middle coil is 890±20℃.

[0017] Step 10: Take samples of the product coil from Step 9 and test the performance and metallographic structure of the beginning and end of the steel coil to ensure that the performance of the beginning and end of the steel coil is qualified.

[0018] In step seven,

[0019] Increasing the thickness of the intermediate billet compensates for the shortcomings in performance improvement achieved in step nine. Experiments show that appropriately increasing the thickness of the intermediate billet can increase the tensile strength and yield strength of the material. Therefore, the second technical measure adopted in this technology to solve the performance defects in the head and tail areas of hot-rolled coils is to increase the thickness of the intermediate billet.

[0020] Research has revealed that the control of intermediate billet thickness is linked to improvements in winding temperature, leading to the following formula for controlling intermediate billet thickness:

[0021] M = m * T / t

[0022] In the formula:

[0023] M: Improved intermediate billet thickness, mm;

[0024] m: Thickness of the intermediate billet before improvement, in mm;

[0025] t: The target value for conventional hot-rolling coiling temperature is 590℃;

[0026] T: The target value of the winding temperature in the beginning and end areas of the hot roll, where T = t + 30℃.

[0027] Replacing T with t in the formula, the above control relationship regarding the thickness of the intermediate billet is as follows:

[0028] M = m * (1 + 30℃ / t).

[0029] In step eight,

[0030] During hot winding, a 30°C temperature compensation at the beginning and end of the coil improves its tensile strength and yield strength. This is achieved by adding 30°C to the conventional winding temperature, thus establishing the control relationship between the beginning and end winding temperatures of the hot coil:

[0031] T = t + 30℃

[0032] Where: T: Target winding temperature value for the beginning and end regions of the hot roll, °C;

[0033] t: The target value for conventional winding temperature during hot winding. In this technology, it is 590℃, with a deviation of ±20℃.

[0034] The mechanism of improving performance by increasing the coiling temperature at the beginning and end of hot-rolled coils: Taking high-titanium steel as an example, the phase transformation and TiN and TiC precipitation processes of the material undergo significant changes during the cooling of the hot-rolled layer. Increasing the coiling temperature can increase the amount of precipitated terms, thereby improving the material's strength. However, this improvement technique has a problem: although it can improve the tensile strength and yield strength of the beginning and end regions of the hot-rolled coil, the improvement is limited, meaning it cannot immediately bring the performance to a satisfactory level. This is because the tensile and yield properties of the beginning and end regions of the hot-rolled coil differ significantly from the standard requirements, especially the tensile strength, which is 70-90 MPa lower than the standard. Therefore, other technical methods must be explored to further improve the performance of the beginning and end regions of the hot-rolled coil during coiling.

[0035] Compared with the prior art, the advantages of the present invention are as follows:

[0036] (1) The technical advantage of the present invention lies in the control of the temperature at the beginning and end of the hot roll. It has both quantitative relationship and expected effect, and the control process is simple and easy to implement.

[0037] (2) In addition, combining the reduction of intermediate billet thickness to improve the head and tail performance of hot-rolled coils is an innovation. In particular, the implementation of this measure in conjunction with the improvement of the head and tail temperatures of hot-rolled coils has significant effects. Furthermore, the control of the intermediate billet thickness has also formed a relationship, in which the winding temperature is cleverly used as a proportion as a coefficient for controlling the intermediate billet thickness. This is an innovation.

[0038] (3) While addressing the issue of substandard performance at the beginning and end of hot-rolled coils, although it is not directly related to steelmaking, it is recognized that controlling inclusions in molten steel is a prerequisite for improving material properties. Therefore, while addressing the aforementioned hot-rolling process...

[0039] At the same time, it also proposed improvement measures for the control of inclusions in molten steel in various steelmaking processes without significantly increasing production costs. Detailed implementation method:

[0040] To enhance understanding of the present invention, detailed descriptions are provided below in conjunction with embodiments.

[0041] Example: A method for solving the problem of substandard performance at the beginning and end of hot-rolled steel coils, the method comprising the following steps:

[0042] Step one: Desulfurization of molten iron to meet desulfurization requirements and slag removal requirements. To improve the performance of hot-rolled coil ends, sulfide inclusions were controlled, with sulfur content controlled to ≤0.005%. The slag removal area of ​​the molten iron should be no less than one-third of the liquid surface.

[0043] Step two involves pouring the desulfurized molten iron, along with scrap steel, into the converter and adding appropriate slag-forming materials for converter steelmaking, refining the steel to the required composition and tapping it at the desired temperature. The purpose of this step, besides ensuring the required chemical composition and temperature of the molten steel, is to control inclusions to improve the performance of the hot-rolled coil ends. Therefore, the converter low-blowing process uses a nitrogen-first, then argon gas stirring method. This ensures a reduction in nitrogen atoms in the molten steel, maintaining its toughness, while also reducing the cost of argon. Aluminum deoxidation is used at tapping, and to control the oxygen content in the molten steel, the oxygen level is required to be below 500 ppm (previously required to be below 600 ppm).

[0044] Step three: The molten steel from the converter enters the LF furnace for refining. The purpose is to desulfurize and raise the temperature to meet the requirements for the next process. To ensure the performance of the hot-rolled coils at both ends, this step further controls inclusions such as sulfides and oxides (because steelmaking involves many processes and takes a long time, further control of inclusions in the molten steel is necessary, so the previous control methods alone are insufficient). The desulfurization dosage needs to be appropriately increased from the original 2200 kg / heat to 2250 kg / heat, thereby controlling the sulfur content to ≤0.003% (originally ≤0.005%).

[0045] Step four involves sending the molten steel from step three to an RH furnace for further processing. This process involves incorporating titanium and manganese alloys, as well as degassing and refining the steel. Besides alloying and vacuum degassing as required, this step also requires careful control of the oxide content in the molten steel. Production monitoring has shown that meeting the original vacuum treatment requirements for the molten steel is beneficial for improving the head and tail properties of the hot-rolled coils. Therefore, the pure degassing time for the molten steel must be greater than 8 minutes, and the vacuum level must be maintained below 26 kPa.

[0046] Step five involves sending the molten steel from step four to the continuous casting process to form slabs. Besides solidifying the steel into slabs for rolling into hot-rolled coils, the most crucial step in this process is controlling inclusions in the molten steel. Measures include proper protective casting to prevent the molten steel from being exposed to air during casting. Argon gas is used throughout the casting process, along with argon blowing through the submerged entry nozzle and stopper rod. The flow rate is controlled to be less than 7.5 L / min (excessive flow will cause large fluctuations in the liquid level in the crystallizer, which can actually introduce inclusions).

[0047] Step six: Send the slab from step five to the hot rolling mill heating furnace to heat the slab to the required rolling temperature. The slab exiting the furnace should be above 1250°C.

[0048] Step 7: Perform initial rolling on the slab from Step 6 to form an intermediate slab of the required thickness, which is 40mm.

[0049] Step 8: The intermediate billet from Step 7 is finished rolled to produce qualified steel coils that meet the user's requirements. The finishing rolling temperature is controlled at 890±20℃.

[0050] Step nine involves coiling the finished rolled material from step eight to form the product steel coil and to optimize the coiling temperature at the beginning and end of the coil, thereby improving the performance of the beginning and end of the coil. The coiling temperature at the beginning and end of the hot coil is controlled at 920±20℃, and the coiling temperature at the middle coil is 890±20℃.

[0051] Step 10: Take samples of the product coil from Step 9 and test the performance and metallographic structure of the beginning and end of the steel coil to ensure that the performance of the beginning and end of the steel coil is qualified.

[0052] Application Examples:

[0053] The implementation method of this technology is illustrated by taking the production of a certain high-titanium steel coil as an example.

[0054] Step 1, the main process parameters for hot metal desulfurization are shown in Table 1.

[0055] Table 1. Main process parameters for desulfurization of molten iron for high-titanium steel

[0056]

[0057] Step 2. Implementation method of converter smelting, the main process parameters required for its smelting are shown in Table 2.

[0058] Table 2. Main process parameters for converter smelting

[0059]

[0060] Step 3, the main process parameters of the LF furnace refining implementation method are shown in Table 3.

[0061] Table 3. Main process parameters for LF furnace refining

[0062]

[0063]

[0064] Step 4, the main process parameters of the vacuum treatment of RH furnace refined molten steel are shown in Table 4.

[0065] Table 4. Main process parameters for vacuum RH furnace treatment

[0066]

[0067] Step 5: The main chemical composition of the hot-rolled coils produced after the molten steel has undergone desulfurization treatment, converter smelting, LF furnace refining, and RH vacuum treatment is shown in Table 5.

[0068] Table 5. Main chemical components of hot-rolled coils (unit: %)

[0069]

[0070] Step 6, the main process parameters for the slab entering the hot rolling mill heating furnace are shown in Table 6.

[0071] Table 6. Main process parameters for slab heating

[0072]

[0073] Step 7: Display of process parameters for intermediate slab thickness during initial rolling after slab heating. The improved intermediate slab thickness (rounded to the nearest integer) is calculated from the intermediate slab thickness control formula: M = m * (1 + 30℃ / t), as shown in Table 7.

[0074] Table 7. Intermediate billet thickness (unit: mm)

[0075]

[0076] Step 8, the main process parameters of the intermediate billet during finishing rolling are shown in Table 8.

[0077] Table 8. Main process parameters for finishing rolling

[0078]

[0079] Step 9: Display of the main process parameters for hot-rolled coils during winding after finishing rolling. Based on the control relationship of the head and tail winding temperatures of the hot-rolled coil: T = t + 30℃, the target values ​​(deviation ±20℃) of the improved head and tail winding temperatures are calculated, as shown in Table 9.

[0080] Table 9. Main process parameters for hot coil winding

[0081]

[0082] Step 10: The main performance of the hot-rolled coil sample before and after the improvement is shown in Table 10.

[0083] Table 10. Comparison of performance before and after the improvement of hot-rolled coil start and end.

[0084]

[0085] As can be seen from the above implementation, increasing the coiling temperature at the beginning and end of the hot coil, and increasing the thickness of the intermediate billet during hot rolling, are two technical measures with a combined effect, which solved the problem of unqualified tensile strength and yield strength at the beginning and end of the hot coil.

[0086] It should be noted that the above embodiments are not intended to limit the scope of protection of the present invention. Equivalent transformations or substitutions made based on the above technical solutions all fall within the scope of protection of the claims of the present invention.

Claims

1. A method for solving the problem of substandard performance at the beginning and end of hot-rolled steel coils, characterized in that, The method includes the following steps: Step one: Desulfurization of molten iron to meet desulfurization requirements and slag removal requirements. To improve the performance of hot-rolled coil ends, sulfide inclusions were controlled, with sulfur content controlled at ≤0.005%, and the slag removal area of ​​the molten iron not less than one-third of the liquid surface. Step two: The desulfurized molten iron, along with the scrap steel, is poured into the converter and mixed with appropriate slag-forming materials for converter steelmaking. The steel is then tapped at the required composition and temperature. Step three: The molten steel from the converter enters the LF furnace for refining. The purpose is to desulfurize and raise the temperature to meet the requirements for the next process. Step four involves sending the molten steel from step three to an RH furnace for further processing. The purpose of this process is to incorporate titanium and manganese alloys, and to degas and refine the steel. The degassing time must be greater than 8 minutes, and the vacuum level must be maintained below 26 kPa. Step five: The molten steel from step four is sent to the continuous casting process to be continuously cast into slabs. Argon gas protection is used throughout the casting process, along with argon gas blowing through the submerged entry nozzle and stopper rod. The flow rate is controlled to be less than 7.5 L / min. Step six: Send the slab from step five to the hot rolling mill heating furnace to heat the slab to the required rolling temperature. The slab exiting the furnace should be above 1250°C. Step seven involves initial rolling the slab from step six to form an intermediate slab of the required thickness, which is 40 mm. Step 8: The intermediate billet from Step 7 is finished rolled to produce qualified steel coils that meet the user's requirements. The finishing rolling temperature is controlled at 890±20℃. Step nine involves coiling the finished rolled material from step eight to form the product steel coil and to optimize the coiling temperature at the beginning and end of the coil, thereby improving the performance of the beginning and end of the coil. The coiling temperature at the beginning and end of the hot coil is controlled at 920±20℃, and the coiling temperature at the middle coil is 890±20℃. Step 10: Take samples of the product coil from Step 9 and test the performance and metallographic structure of the beginning and end of the steel coil to ensure that the performance of the beginning and end of the steel coil is qualified.

2. The method for controlling high surface area cold-rolled hot-dip aluminized zinc plating according to claim 1, characterized in that, In step 7, Increasing the intermediate billet thickness, and controlling the intermediate billet thickness in conjunction with improvements in the winding temperature, results in the following control formula for the intermediate billet thickness: M = m * T / t In the formula: M: Improved intermediate billet thickness, mm; m: Thickness of the intermediate billet before improvement, in mm; t: The target value for conventional hot-rolling coiling temperature is 590℃; T: The target value for the winding temperature of the beginning and end regions of the hot roll, where T = t + 30℃. Replacing T with t in the formula, the above control relationship regarding the thickness of the intermediate billet is as follows: M = m * (1 + 30℃ / t).

3. The method for controlling high surface area cold-rolled hot-dip aluminized zinc plating according to claim 1, characterized in that, In step 9, During hot winding, a 30°C temperature compensation at the beginning and end of the coil improves its tensile strength and yield strength. This is achieved by adding 30°C to the conventional winding temperature, thus establishing the control relationship between the beginning and end winding temperatures of the hot coil: T = t + 30℃ Where: T: Target winding temperature value for the beginning and end regions of the hot roll, °C; t: The target value for conventional winding temperature during hot winding. In this technology, it is 590℃, with a deviation of ±20℃.

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